You've probably read in the instructions that this unit switches off after a few minutes of inactivity. You must press a button to restart.
If that's not the issue, you'll need to move the sensor so that the magnet almost touches it as it passes. The instructions say the gap should be 1mm. I usually move the sensor until the magnet just hits as the wheel spins, then move it away just a little. You may need to put some shims under the sensor to move it closer. If it still does not work, the sensor could be bad or the wire could be bad.
I've never had one that didn't function out of the box, but It's a good idea to test units before installing. Simply hold the sensor in one hand (start the unit, if necessary) and pass the magnet (closely) over the sensor a few times. The unit should display speed, etc.